Plastic optical materials are lightweight, hardly breakable, and excellent in dyeability as compared with optical materials made of inorganic materials such as glass. Therefore, plastic materials are widely used as optical materials for eyeglass lenses, camera lenses, and the like. In recent years, there has been an increased demand for higher performance of optical materials, particularly in terms of high refractive index, high Abbe number, low specific gravity, high heat resistance, high impact resistance, and the like.
The higher the sulfur content in a plastic optical material, the higher the refractive index thereof. Thus, various attempts have been made to add sulfur atoms to the raw material monomers. Due to the binding nature of sulfur atoms, however, it has not been solved to introduce a large number of sulfur atoms in the same molecule.
For this purpose, a polythiourethane-based optical material has been prepared by polymerizing a polythiol containing a thiol group (—SH) at its terminal ends with an isocyanate compound. But it is difficult to contain a sufficient amount of sulfur through the sulfur atoms at the terminal ends only. Thus, a technique of incorporating a thioether group (—C—S—C—) into a molecule has been used together therewith.
In order to incorporate a thioether group into a molecule, a method of reacting a metal monosulfide such as Na2S.xH2O (x=5 or 9) with two organic halides has been generally used.